[CogManager, Utils] Handle missing cogs correctly, add some helpful algorithms (#1989)

* Handle missing cogs correctly, add some helpful algorithms

For cog loading, only show "cog not found" if the module in question was the one
that failed to import. ImportErrors within cogs will show an error as they should.

- deduplicator, benchmarked to be the fastest
- bounded gather and bounded async as_completed
- tests for all additions

* Requested changes + wrap as_completed instead

So I went source diving and realized as_completed works the way I want it to,
and I don't need to reinvent the wheel for cancelling tasks that remain
if the generator is `break`ed out of. So there's that.

redbot/core/cog_manager.py

@@ -3,9 +3,10 @@ import pkgutil
 from importlib import import_module, invalidate_caches
 from importlib.machinery import ModuleSpec
 from pathlib import Path
-from typing import Tuple, Union, List
+from typing import Tuple, Union, List, Optional
 
 import redbot.cogs
+from redbot.core.utils import deduplicate_iterables
 import discord
 
 from . import checks, commands
@@ -18,12 +19,13 @@ from .utils.chat_formatting import box, pagify
 __all__ = ["CogManager"]
 
 
-def _deduplicate(xs):
-    ret = []
-    for x in xs:
-        if x not in ret:
-            ret.append(x)
-    return ret
+class NoSuchCog(ImportError):
+    """Thrown when a cog is missing.
+
+    Different from ImportError because some ImportErrors can happen inside cogs.
+    """
+
+    pass
 
 
 class CogManager:
@@ -56,7 +58,7 @@ class CogManager:
         conf_paths = [Path(p) for p in await self.conf.paths()]
         other_paths = self._paths
 
-        all_paths = _deduplicate(list(conf_paths) + list(other_paths) + [self.CORE_PATH])
+        all_paths = deduplicate_iterables(conf_paths, other_paths, [self.CORE_PATH])
 
         if self.install_path not in all_paths:
             all_paths.insert(0, await self.install_path())
@@ -209,11 +211,10 @@ class CogManager:
 
         Raises
         ------
-        RuntimeError
-            When no matching spec can be found.
+        NoSuchCog
+            When no cog with the requested name was found.
         """
-        resolved_paths = _deduplicate(await self.paths())
-
+        resolved_paths = await self.paths()
         real_paths = [str(p) for p in resolved_paths if p != self.CORE_PATH]
 
         for finder, module_name, _ in pkgutil.iter_modules(real_paths):
@@ -222,9 +223,11 @@ class CogManager:
                 if spec:
                     return spec
 
-        raise RuntimeError(
-            "No 3rd party module by the name of '{}' was found"
-            " in any available path.".format(name)
+        raise NoSuchCog(
+            "No 3rd party module by the name of '{}' was found in any available path.".format(
+                name
+            ),
+            name=name,
         )
 
     @staticmethod
@@ -246,16 +249,24 @@ class CogManager:
             When no matching spec can be found.
         """
         real_name = ".{}".format(name)
+        package = "redbot.cogs"
+
         try:
-            mod = import_module(real_name, package="redbot.cogs")
+            mod = import_module(real_name, package=package)
         except ImportError as e:
-            raise RuntimeError(
-                "No core cog by the name of '{}' could be found.".format(name)
-            ) from e
+            if e.name == package + real_name:
+                raise NoSuchCog(
+                    "No core cog by the name of '{}' could be found.".format(name),
+                    path=e.path,
+                    name=e.name,
+                ) from e
+
+            raise
+
         return mod.__spec__
 
     # noinspection PyUnreachableCode
-    async def find_cog(self, name: str) -> ModuleSpec:
+    async def find_cog(self, name: str) -> Optional[ModuleSpec]:
         """Find a cog in the list of available paths.
 
         Parameters
@@ -265,23 +276,16 @@ class CogManager:
 
         Returns
         -------
-        importlib.machinery.ModuleSpec
-            A module spec to be used for specialized cog loading.
-
-        Raises
-        ------
-        RuntimeError
-            If there is no cog with the given name.
+        Optional[importlib.machinery.ModuleSpec]
+            A module spec to be used for specialized cog loading, if found.
 
         """
-        with contextlib.suppress(RuntimeError):
+        with contextlib.suppress(NoSuchCog):
             return await self._find_ext_cog(name)
 
-        with contextlib.suppress(RuntimeError):
+        with contextlib.suppress(NoSuchCog):
             return await self._find_core_cog(name)
 
-        raise RuntimeError("No cog with that name could be found.")
-
     async def available_modules(self) -> List[str]:
         """Finds the names of all available modules to load.
         """